1. Field of the Invention
The present invention relates to those materials known in the trade as oriented silicon-irons and which normally contain about 3% silicon, about 0.1% manganese, between about 0.02 and about 0.03% sulfur and a balance essentially iron with incidental impurities. These alloys when properly processed are capable upon heat treatment of developing a preferred grain orientation which is known in terms of Miller indicies as (110) [001], which orientation has been described generically as "cube-on-edge".
2. Description of the Prior Art
The core materials which are employed in transformers today comprise an alloy containing nominally about 3.25% silicon and which has been processed to obtain a preferred grain orientation. The grain orientation, which is accomplished during the final heat treatment, has been characterized as an orientation resulting from the application of internal energy to certain preferentially aligned grain nuclei. When fully processed to obtain the desired orientation, the major portion of the grains have an orientation which has been described in terms of Miller indices as (110) [001] orientation or as a cube-on-edge orientation.
During final heat treatment of this material, the heat treatment takes place while the material is in a coil configuration. It is essential during such heat treatment that the individual convolutions of the coil must be provided with a material to insulate or separate the turns from one another since the heat treatment temperatures involved are sufficiently high that sticking or welding of the adjacent convolutions would occur except for the insulation provided thereto. This insulation has usually taken the form of a magnesium oxide coating which has been applied to the surface of the steel. During said final heat treatment some of the MgO reacts with the components of the surface of the steel including iron, oxygen and silicon to form a complex, tightly adherent layer of a magnesium silicate type glass coating on the surface of the steel. This tenacious film will permit the unwinding of the coil material by preventing adhesion of the adjacent convolutions of the coil. In addition, the glass which forms during said high temperature heat treatment also acts as an excellent electrical insulator which is highly useful when the material is used subsequently in electrical apparatus in the form of a stacked lamination or a wound core.
Attempts to improve the observed magnetic characteristics of the prior art material have usually been limited to the aspect of improvement in the core loss exhibited by the materials because the saturation induction value of the alloy material is essentially limited by its chemical composition. These attempts at improving the core loss have centered about the development of preferred orientations, higher purity content and alloying variations which tend to alter the effective resistance exhibited by the alloy.
It has been discovered that the prior art magnetic material and in particular the prior art silicon steel having a reacted MgO coating on the surface thereof, does not exhibit an optimum low core loss especially where the material is operated at high inductions. This is predicated on the fact that the magnesium silicate coating which forms during the high temperature annealing may be a factor in causing unfavorable stresses to be imparted to the underlying metal thereby adversely affecting the observed core loss. Other reasons which have been advanced for the non-optimum core loss at high operating inductions include the aspect of the interaction of the coating with the surface of the metal during high temperature heat treatment may be harmful and the coating which is formed may be an effective deterrent for removing certain impurities, particularly carbon, to the desired low level during said high temperature heat treatment operation.
In order to alleviate these factors the present invention employs a coating which is non-reactive with the surface of the steel and promotes purification of the alloy during said high temperature heat treatment operation particularly the decarburization of the underlying metal.